The invention relates generally to electric and hybrid vehicles, and more specifically to a vehicle propulsion system having a continuously variable transmission (CVT).
As is known in the art, CVTs are capable of continuously or steplessly varying the speed of rotation of an output shaft over a range of operating speeds while a high speed or input shaft rotates. CVTs may be used in automotive applications as a substitute for conventional fixed-ratio transmissions or multi-geared automatic transmission systems. CVTs have an advantage over conventional transmissions because of their large and “continuous” transmission ratio coverage, which may range from 1:10 to 1:20 for example. In order to achieve this large range of gear ratios, such CVTs tend to be large and technically complex and are therefore expensive to manufacture. CVTs are typically less efficient than conventional gearboxes due to additional friction between the input and output shafts, and the costs and inefficiencies increase with size. However, CVTs typically lessen noticeable shifting events, provide a smoother and more comfortable ride because of continuous traction torque, and allow an operation point of the traction engine to shift to more economic operating points, thus increasing fuel efficiency of the engine compared with vehicles equipped with fixed-gear, automatic transmissions.
It would therefore be desirable to design a vehicle propulsion system that incorporates a CVT while reducing the size, cost, and inefficiencies typically associated with CVTs.